The invention also concerns the application of such a control head for the embodiment of several successive controls on parts whose shape evolves during production at clearly defined points of these parts.
The use of parts made of a composite material has been considerably expanded over the last few years, especially as regards aeronautic, spatial, automobile, maritime and railway applications. This development is principally explained by virtue of the considerable lightening of structures it allows for and by the possibility offered by these materials so as to obtain complex shapes by means of moulding.
Parts made of a composite material are most often produced by the flat stretcher-levelling of several superimposed laps or layers, each formed of fibers impregnated with resin. Depending on the application involved, the fibers may be made of carbon, glass, Kevlar, etc. The resin constituting the matrix of the composite material is usually a duroplastic resin which polymerizes with the temperature. In particular, this polymerization may be obtained in a subsequent moulding stage during which the part is given its definitive shape.
Despite the afore-mentioned advantages, parts made of a composite material may have certain defects resulting from the particular structure of these materials. These defects originate from the lack of cohesion between the fibers resulting in the presence of air or gas pockets, possibly extensive ones. These are expressed on finished parts, either by a certain porosity of these parts when the pockets are small and distributed throughout the entire volume, or by a phenomenon, known as delamination, when the large air pockets are formed between the adjacent layers of the material.
The detection of these defects on finished parts is an operation required for the approval of these parts.
Furthermore, it is desirable to be able during production to monitor the evolution of defects and of the thickness of the part as its shape gradually varies. In fact, this monitoring makes it possible to optimize the conception of the starting structure of the flat stretch-levelled part.
So as to carry the non-destructive control of parts made of a composite material, original techniques have been developed, as conventional techniques for controlling metal parts, such as eddy current techniques, were not able to be applied to such parts. These techniques for controlling parts made of a composite material basically rely on the use of ultrasonic probes or transducers usually functioning at a frequency of between 0.5 and 15 MHz.
In particular and in the case of controlling parts with complex shapes, a reflected wave method is used in which the ultrasonic transducer is used both as a transmitter and a receiver. With the transducer being placed opposite the surface of the part, it receives in return the echoes sent back by this surface and by the surface opposite the part, as well as the echoes resulting from a possible delamination. Thus, it is possible to punctually control the thickness of the part and to detect the presence of delamination defects, as well as the position and extent of these defects. In addition, the measurement of the attenuation of the echo sent back by the surface opposite the part, corresponding to ultrasonic waves having twice traversed the thickness of said part, makes it possible to know the porosity of the structure.
As with all the ultrasonic control techniques, this reflected wave method has the drawback of being a punctual method so that the control of a large surface with the aid of a single transducer is a long and fastidious operation. If it is sought to reduce the control time and to group several transducers when a flat part or one with a uniform curve is being controlled, this objective has not been possible up until now when this involves controlling parts having a non-uniform shape, this frequently being the case with parts made of a composite material whose specific advantage is to make it possible to produce such parts.
In effect, the reflected wave control technique requires a relatively precise positioning of the transducers perpendicular to the surface of the part so that the echoes sent back by the surface of the part are properly received by the transducer. If this requirement may be satisfied relatively easily for a single transducer or for a group of transducers placed in relation to a flat surface or one with a uniform curve, this poses a problem which has never been resolved up until now when it is sought to combine several transducers so as to control parts having a non-uniform shape.
An even greater difficulty currently exists making it impossible to control, with the aid of a given control head comprising several transducers, a part whose shape evolves while being produced. In fact, along with the need for adaptation to the evolution of the shape of the part, there is a further difficulty of carrying out controls at points which remain unchanged whenever the shape changes.